The benefits of ceramic filter media for specific uses of water filtering are well established. Ceramic filter media provides acceptable filtration and has a long life relative to other filter media. There have been, however, many challenges to producing economically a high quality ceramic filter for water filters, such as portable water filters, home water filters, and water filters for industrial use. A number of design criteria must be satisfied in order to produce a high quality ceramic filter media suitable for water filtration. For many types of water filtration, a high level of dimensional control must be maintained. The length of the filter media must be precisely cut for use in the particular filter media housing. The inside and outside diameters must be within a narrow tolerance range. This becomes particularly important where, as with ceramic filter media, the media is cleaned by abrasion, and the useful life of the filter media is determined by the external dimensions of the ceramic filter.
One traditional method utilized in manufacturing ceramic filter media is a slip cast method. This method involves the use of a porous mold (commonly made out of gypsum), into which a slurry containing ceramic raw materials is directed. The slurry is generally either a water-based or alcohol-based slurry. Once the slurry has been introduced into the mold, the liquid portion of the slurry is drawn into the mold. A filter cake is formed on the wall of the mold. Upon removal of the structure that remains, including the filter cake, from the mold, the structure is fired in a kiln and a ceramic filter results. This method involves, however, significant manufacturing costs and is unable to achieve high levels of precision in terms of tolerance ranges.
Another method of manufacturing a ceramic filter involves use of ceramic powder. Ceramic powder beads are introduced to pressure and heat through a sintering process. The particles coalesce and a pore structure results. The structure is fired and dipped in an acid. The difficulty in respect to this process relates to the percent porosity. This process reduces the porosity of the filter media and thereby slows down the flow of water through the filter media.
The nature of extrusion processes has heretofore prevented effective manufacturing of ceramic filters for use in water filtering devices. A number of problems have persisted that have prevented use of this technique in manufacturing ceramic filter media using diatomaceous earth. A first major problem involves the need to maintain the desired shape of the extruded material. For portable water filtration, a tubular shape is understood to be the most effective for filtration and longevity of the filter media. The primary constituent in a sintered ceramic filter is silica (SiO.sub.2) which is highly abrasive and therefore difficult to pass through an extruding machine. This, combined with the fact that the silica must be urged through a small opening, creates substantial problems with respect to maintaining a particular desired shape, such as a tube. Because of the abrasiveness of silica, the appropriate mixture of non-ceramic, extrusion-aid materials must be utilized and carefully balanced to enable extrusion. If the mixture including extrusion-aid and ceramic raw materials is too dry, the material becomes difficult to extrude. Furthermore, even if it were possible to extrude a particularly dry mixture of ceramic materials, the pores would likely be too small and too compressed to allow proper filtration. On the other hand, a solution that is too aqueous will result in an extruded structure that does not maintain its proper shape.
A second major problem with the extrusion process relates to maintaining the relative desirable characteristics of the constituent materials. The mixture of constituents that will result in a proper ceramic filter media must be mixed a particular way to achieve the desired density of materials. In the extrusion process, the material mixture must flow continuously through the extruder without allowing air pockets to be introduced into the material. Otherwise, flaws may be created in the resulting ceramic filter media.
A third problem relates to the pore size and pore distribution in the resulting filter media. For proper filtration to occur, pore distribution must be even about the cylindrical wall. In addition, the pore size must be proper to enable filtration at an optimum level. Special problems are presented when drying tubular filter media in order to maintain proper pore size and distribution.
As mentioned above, maintaining dimensional controls to ensure that the resulting ceramic filter media falls within an acceptable range of tolerances is essential. Provided the foregoing problems with respect to the extrusion process can be overcome, as indeed is the case in the present invention, utilizing the extrusion process to produce filter media will provide the acceptable dimensional control and significantly reduce production costs.
Another primary advantage of using an extrusion process for manufacturing sintered diatomaceous ceramic filters is the uniformity of material. Because the constituent materials are thoroughly mixed prior to extrusion, the filter media material that results as highly uniform and is highly suitable for filtering water.
Another advantage to utilizing extrusion in the process of manufacturing ceramic water filters is that the resulting filter media requires no secondary machining. In contradistinction, the filter media produced by the slipcast method requires machining and milling in order to achieve acceptable tolerance ranges depending upon the end uses for the filter, such as portable water filtration cartridges which require a measurement gage to determine the useful life of the filter media. Furthermore, extrusion requires fewer manufacturing steps than slipcasting. Extrusion also provides the ability to manufacture a wide variety of filter media lengths and sizes with little expense. The end result is that it would be much less expensive to manufacture a sintered diatomaceous ceramic filter using an extrusion process as compared to prior known processes such as slipcasting.
In view of the foregoing, there is a substantial need to produce a ceramic filter media through use of an extrusion process so that suitable dimensional control and other desired characteristics of filter media can be maintained and to reduce the cost of manufacturing the ceramic filter media.